In addition to their striking color, blood oranges are believed to have significant health-promoting properties, combining the high content of vitamin C, carotenoids and fiber of common blond oranges with the health-promoting properties of anthocyanin pigments (de Pascual-Theresa et al., 2010; Paradez-Lopez et al., 2010; Davies, 2007; Prior and Wu, 2006). The high anthocyanin content of blood oranges underpins their high antioxidant activity (Proteggente et al., 2011; Kelebek et al., 2008; Jayaprakasha and Patl, 2007; Rapisada et al., 1999). Consumption of blood orange juice has been shown to reduce oxidative stress in diabetic patients (Bottina et al., 2002), protect DNA against oxidative damage (Guarnieri et al., 2007; Riso et al., 2005) and may reduce cardiovascular risk factors more generally, as demonstrated for other high-anthocyanin foods (de Pascual-Theresa et al., 2010; Paradez-Lopez et al., 2010; Toufektsian et al., 2008). Recently, blood orange juice has been shown to limit the development of adipocytes and weight gain in mice and to confer resistance to obesity compared to blond orange juice or water (Titta et al., 2010). In a mouse model of obesity, blood orange juice consumption rescued almost completely the transcriptional reprogramming induced by a high fat diet.
Despite increasing consumer interest in their high nutritional quality, blood oranges do not have a global market, largely due to a lack of dependability of color development. All blood orange varieties require strong day-night thermal clines for intense color formation in fruit flesh, and varieties such as Moro, with the potential for high pigmentation, are strongly dependent on the prevailing climatic conditions during fruit ripening for full colour development. Post-harvest storage of fruit in the cold enhances pigmentation, but this is an expensive measure to ensure high levels of pigmentation, and can increase post-harvest losses (Crifò et al., 2011; Latado et al., 2008; Rapisada et al., 1999). The dependency of anthocyanin accumulation on environment means that the most reliable blood orange production, on a commercial scale, is limited to Italy, specifically to the Sicilian area around Mount Etna (Zarba and Pulvirenti, 2006) where it remains highly seasonal. Although blood oranges are grown in other countries, in some years entire harvests are lost due to non-optimal conditions during ripening of fruit, and when they are cultivated in Brazil or Florida (the largest producers of oranges worldwide), coloration is generally weak or absent and unreliable (Latado et al., 2008; Hodgson, 1967). To ensure a stable supply of blood oranges, improved oranges trees which can produce blood oranges with reliably high levels of anthocyanins under a variety of environmental conditions, are desired.
Anthocyanins are natural pigments found typically in red, purple and blue fruit and flowers (Winkel-Shirley, 2001). Many varieties of blood orange have been derived from old Italian varieties such as Doppio Sanguigno and include more recently-derived varieties such as Tarocco and Moro, which generally have higher levels of anthocyanin pigmentation of their fruit (FIG. 1). The history of these varieties is debated although authoritative texts suggest three independent derivations: one Italian/Sicilian from Doppio Sanguigno/Maltaise Sanguine, a second in Spain from Doblefina and a third from Shamouti Orange referred to as Shamouti Blood or Palestinian Blood Jaffa Orange (Hodgson, 1967). In the mid-19th century it was believed that blood oranges arose by bud mutation in the Mediterranean region following the introduction of sweet orange in the 16th century (Holmes, 1924). More recently it was suggested that blood oranges originated much earlier in Asia (Hodgson, 1967; Chapot, 1963). The blood orange was first documented in Italy in ‘Hesperides’ by Ferrari (1646), a Jesuit scholar who wrote of an orange with purple-colored flesh that tasted strangely like a grape (Ferrari, 1646). Ferrari suggested that the blood orange was brought to Sicily by a Genoese missionary after a long journey which started in China. However, claims of Chinese poems referring to scarlet/red oranges, dating from the Tang period to more recent times, are probably based on mistranslation of the term ‘orange’ and likely refer to mandarins which never accumulate anthocyanins. Some definitively red oranges are portrayed in a picture by Bartolomeo Bimbi, a Florentine artist who painted the Medici Citrus collections early in the 18th century.